Hello,
Hoping someone here might have an idea. I have an (OLD) Peavey Musician Guitar Head. Technically I guess it's a Mark I, but it actually predates the Mark name. It's a series 400GH. Bought new in 1980 and used (very little gigging) through the mid-late 80s. Never a problem with it then. Sat on the shelf for the last 20-25 years.
I pulled it down recently to use/leave at practice. Plugged it in and it began to smoke. Brought it home, took the heat sink/circuit board out (separate install from the transformers and the control panels) and vacuumed out the dust. Found a small piece of aluminum (tab off the bottom of an old soda cap) was lodged against the transformer. Removed it, reassembled and plugged in. No Smoke. Yay. Plugged in a speaker cable to speaker cab and immediately got a huge buzz. Instrument sounds DID come through over the buzz.
Tried various cables, outlets etc. No change. Know the speakers work - just played a gig with them a couple months ago. Listened closely with everything unplugged and the amp on. The amp ITSELF seems to be humming/buzzing. Not loud - ear to the box kind of thing. But it's there.
Any ideas? I'm assuming a ground short, but not sure instrument would come through cleanly over the buzz if that was the case. Transformer problem? Box was never opened before and I didn't pull anything loose.
Any help would be appreciated.
Thanks
Nick
Sounds like the power supply caps are shot. Replace them and you should be good to go.
Or...
Your amp may be making DC. Look at the speaker cone when you turn the amp on. Does it move one direction and stay there? If so, your amp is puting DC voltage on the output. TURN IT OFF IMMEDIATELY AND DO NOT CONNECT A SPEAKER UNTIL IT IS FIXED.
If the speaker stays at rest position, then at least your outputs are not blown. IN that case it may indeed just be filter caps.
The filter caps in those old Peaveys don;t usually fail of a sudden, they usually just get noisier and noisier. But anything is possible. What is also common, and more likely in sudden hum situationbs is cracked solder or loose connection to one or both main filters. FIlter caps don't usually smoke or cause smoke.
Quote from: Enzo on August 23, 2012, 07:20:02 PM
FIlter caps don't usually smoke or cause smoke.
They can smoke...once. Also, you made some good points and I'm actually starting to think its the output transistors from his description. I've mostly only repaired chipamp setups but when the power amp chip fails it sometimes does what he is describing. I jumped to the capacitor solution due primarily to the age of the amp (caps can fail with age) and that might have been too quick a jump.
Thanks guys.
I won't have a chance to look at it again until this week sometime. I'll check the speaker movement for DC and, as I'll have more time than the other day, I'll take it back apart and check for anything that looks loose, cracked etc.
One thing I forgot to mention the other day (and not sure if it's of vallue), but the buzz comes from either output. This amp has a direct out which delivers 300W into 2/4 Ohms and a X-Over out that delivers 200W into 2/4 Ohms. I might have those figures backwards, but the point is, it's a separate output channel since it runs through a x-over transformer, right? Anyway, as I said, not sure that adds anything.
The smoke, btw, wasn't some huge wildfire. it was more a single source point, like a cigarette inside the case, so I'm guessing that was a component burning. And it may well have been 20 years of dust on a hot point.
I'll report back later in the week after a better exam and the DC test.
Thanks again
Nick
AFAIK that amp has a single power amp, although probably preamp signal can be routed straight to it or through an electronic crossover.
Anyway your amp is probably shorted and must be repaired.
Smoke - sorry, but it's unlikely that it's something harmless like "dust on a hot point". With the power off give the internals a very good looking over with a bright light and magnifying glass, particularly around the area of the origin of the smoke. It's highly likely that you will find a roasted resistor, and identifying this will provide a clue as to what is wrong with your amp. In my experience smoke like this normally means you have a dead transistor or three that will have to be located and replaced.
To prevent more smoke, make up a limiting lamp for future testing;
http://www.ssguitar.com/index.php?topic=2093.0 (http://www.ssguitar.com/index.php?topic=2093.0)
What I think is the circuit is attached.